Charged balanced devices with shielded gate trench

ABSTRACT

This invention discloses a semiconductor power device disposed on a semiconductor substrate includes a plurality of deep trenches with an epitaxial layer filling said deep trenches and a simultaneously grown top epitaxial layer covering areas above a top surface of said deep trenches over the semiconductor substrate. A plurality of trench MOSFET cells disposed in said top epitaxial layer with the top epitaxial layer functioning as the body region and the semiconductor substrate acting as the drain region whereby a super junction effect is achieved through charge balance between the epitaxial layer in the deep trenches and regions in the semiconductor substrate laterally adjacent to the deep trenches. Each of the trench MOSFET cells further includes a trench gate and a gate-shielding dopant region disposed below and substantially aligned with each of the trench gates for each of the trench MOSFET cells for shielding the trench gate during a voltage breakdown.

This Patent Application is a Divisional Application and claims thePriority Date of a co-pending application Ser. No. 13/669,267 filed onNov. 5, 2012. Application Ser. No. 13/669,267 is a DivisionalApplication of another application Ser. No. 12/932,172 filed on Nov. 6,2011 by common inventors of this Application and now issued into U.S.Pat. No. 8,304,312. Application Ser. No. 12/932,172 is a DivisionalApplication of application Ser. No. 12/321,435 filed on Jan. 21, 2009 bycommon inventors of this Application now issued as U.S. Pat. No.7,893,488. This application is further a Continuation in Part (CIP)Application of a co-pending Application with a Ser. No. 12/229,250 filedby a common Inventor of this Application on Aug. 20, 2008. Thedisclosures made in the application Ser. Nos. 12/321,435, 12/932,172 and12/229,250 are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to vertical semiconductor power devices.More particularly, this invention relates to configurations and methodsimplemented with a single thin epitaxial layer with improvedmanufacturability for manufacturing flexibly scalable charge balancedvertical semiconductor power devices with a super-junction structure andshielded gate trench with simple manufacturing processes flexiblyadjustable for applications to different targeted breakdown voltages.

2. Description of the Prior Art

Conventional manufacturing technologies and device configuration tofurther increase the breakdown voltage with reduced series resistanceare still confronted with manufacturability difficulties. The practicalapplications and usefulness of the high voltage semiconductor powerdevices are limited due to the facts that the conventional high powerdevices generally have structural features that require numeroustime-consuming, complex, and expensive manufacturing processes. As willbe further discussed below, some of the processes for manufacturing thehigh voltage power devices are complicated thus having low throughputand low yields. Furthermore, instead of using a raw semiconductor wafer,the semiconductor power devices are generally fabricated withpreprocessed wafer formed with an epitaxial layer thereon. Theproduction costs of the semiconductor power devices are thereforeincreased. Also, the functionality and performance characteristics areoften dictated by the process parameters applied in forming thisepitaxial layer. For these reasons, the application of such preprocessedwafers further limits the manufacturability and the productionflexibility of the power devices that are now dependent on the originalpreprocessed wafer employed to manufacture the semiconductor powerdevices.

In comparison to conventional technologies, the super-junctiontechnologies have advantages to achieve higher breakdown voltage (BV)without unduly increasing the drain-to-source on resistance, Rdson. Forstandard power transistor cells, breakdown voltage is supported largelyon the low doped drift layer. Therefore, the drift layer is made withgreater thickness and more resistive at higher voltage ratings. Howeverthis has the effect of greatly increasing the Rdson. In conventionalpower devices, the Rdson has approximately a functional relationshiprepresented by:Rdson∝BV^(2.5)In contrast, a device having a super-junction configuration isimplemented with a charge balanced drift region. The resistance Rdsonhas a more favorable functional relationship with the breakdown voltage.The functional relationship can be represented as:Rdson∝BVFor high voltage applications, it is therefore desirable to improve thedevice performance by designing and manufacturing the semiconductorpower devices with super-junction configurations for reducing theresistance Rdson while achieving high breakdown voltage. Regionsadjacent to the channel within the drift region are formed with anopposite conductivity type. The drift region may be relatively highlydoped, so long as the regions adjacent to the channel are similarlydoped but of an opposite conductivity type. During the off-state, thecharges of the two regions balance out such that the drift regionbecomes depleted, and can support a high voltage. This is referred to asthe super-junction effect. During the on-state, the drift region has alower resistance Rdson because of a higher doping concentration.

However, conventional super-junction technologies still have technicallimitations and difficulties when implemented to manufacture the powerdevices. Specifically, multiple epitaxial layers and/or buried layersare required in some of the conventional structures. Multiple etch backand chemical mechanical polishing (CMP) processes are necessary in manyof the device structures according to the previous manufacturingprocesses. Furthermore, the manufacturing processes often requireequipment not compatible with standard foundry processes. For example,many standard high-volume semiconductor foundries have oxide CMP(chemical mechanical polishing) but do not have silicon CMP, which isrequired for some superjunction approaches. Additionally, these deviceshave structural features and manufacturing processes not conducive toscalability for low to high voltage applications. In other words, someapproaches would become too costly and/or too lengthy to be applied tohigher voltage ratings. As will be further reviewed and discussionsbelow, these conventional devices with different structural features andmanufactured by various processing methods, each has limitations anddifficulties that hinder practical applications of these devices as nowdemanded in the marketplace.

A conventional type of semiconductor power device for high voltageapplications includes those devices formed with standard structures asdepicted in FIG. 1A for a standard VDMOS that do not incorporate thefunctional feature of charge balance. For this reason, there is nobreakdown voltage enhancement beyond the one-dimensional theoreticalfigure of merit, i.e., the Johnson limit, according to the I-V(current-voltage) performance measurements and further confirmed bysimulation analyses of this type of devices. The devices with thisstructure generally have relatively high on-resistance due to the lowdrain drift region doping concentration in order to satisfy the highbreakdown voltage requirement. In order to reduce the on resistanceRdson, this type of device generally requires large die size. Despitethe advantages that the devices can be manufactured with simpleprocesses and low manufacturing cost, these devices are however notfeasible for high current low resistance applications in the standardpackages due the above discussed drawbacks: the die cost becomesprohibitive (because there are too few dies per wafer) and it becomesimpossible to fit the larger die in the standard accepted packages.

A second type of devices includes structures provided withtwo-dimensional charge balance to achieve a breakdown voltage higherthan the Johnson limit for a given resistance, or a lower specificresistance (Rdson*Area product) than the Johnson limit for a givenbreakdown voltage. This type of device structure is generally referredto as devices implemented with the super junction technology. In thesuper junction structure, a charge-balance along a direction parallel tothe current flow in the drift drain region of a vertical device, basedon PN junctions and field plate techniques implemented in oxide bypasseddevices to enable a device to achieve a higher breakdown voltage.

FIG. 1B is a cross sectional view of a device with super junction toreduce the specific resistance (Rsp, resistance times active area) ofthe device by increasing the drain dopant concentration in the driftregion while maintaining the specified breakdown voltage. The chargebalance is achieved by providing P-type (for n-channel devices) verticalcolumns formed in the drain to result in lateral and complete depletionof the drain at high voltage to thus pinch off and shield the channelfrom the high voltage drain at the N+ substrate. Such technologies havebeen disclosed in Europe Patent 0053854 (1982), U.S. Pat. No. 4,754,310,specifically in FIG. 13 of that Patent, and U.S. Pat. No. 5,216,275. Inthese previous disclosures, the vertical super junctions are formed asvertical columns of N and P type dopant. In vertical DMOS devices, thevertical charge balance is achieved by a structure with sidewall dopingto form one of the doped columns as were illustrated in drawings. Inaddition to doped columns, doped floating islands have been implementedto increase the breakdown voltage or to reduce the resistance asdisclosed by U.S. Pat. No. 4,134,123 and U.S. Pat. No. 6,037,632. Suchdevice structure of super junction still relies on the depletion of theP-regions to shield the gate/channel from the drain. The floating islandstructure is limited by the technical difficulties due to charge storageand switching issues.

For super junction types of devices as described above, the method ofmanufacturing are generally very complex, expensive and require longprocessing time due to the facts that the methods require multiple stepsand several of these steps are slow and have a low throughput.Specifically, the steps may involve multiple epitaxial layers and buriedlayers. Some of the structures require deep trenches through the entiredrift region and require etch back or chemical mechanical polishing inmost these processes. For these reasons, the conventional structures andmanufacture methods are limited by slow and expensive manufacturingprocesses and are not economical for broad applications.

In U.S. application Ser. No. 12/005,878 filed by the inventor of thisapplication, of which this application is a Continuation-In-Part, asuper junction device with charge-balancing epitaxial columns grown indeep trenches is disclosed. Trench metal oxide semiconductor fieldeffect transistors (MOSFET) are formed in the top epitaxial layer grownover the deep trenches and the areas surrounding the deep trenches.However the trench gate of this device may experience high electricfields and may be vulnerable to damage during voltage breakdowns.

Thus, in addition to the demands to improve the configurations and themethods of manufacture for the super-junction devices, there is also arequirement to shield the sensitive gates during the breakdown for theactive cells. FIGS. 1C-1 to 1C-3 show the devices disclosed in U.S. Pat.No. 6,635,906 with devices formed with P-floating islands 1 in the bulklayer of the epitaxial layer. These P-floating islands are however notself-aligned to the gate or to trenches and have less effectivenessduring the voltage breakdown in protecting the sensitive trench gates.FIG. 1D shows a figure disclosed by Takaya et. al., in their paper“Floating Island and Thick Bottom Oxide Trench Gate MOSFET (FITMOS),” atthe 17th International Symposium on Power Semiconductor Devices & IC'sin 2005, showing the floating P-regions implanted for charge balancingthe drain and the P-region at the bottom of the trench gates are appliedto separate the gate from the P-region. However, these p-implant regionsbelow the trench gates are in contact with the gate trenches with athick bottom oxide and may reduce the amount of current that can passthrough during on operation.

Therefore, a need still exists in the art of power semiconductor devicedesign and manufacture to provide new device configurations andmanufacturing method in forming the power devices such that the abovediscussed problems and limitations can be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved device structure and manufacturing method to form the dopedcolumns in the drift regions for charge balance with simple andconvenient processing steps. There are no etch-backs or CMP (chemicalmechanical polishing) required thus reducing the processing steps andcan be implemented with just forming a single thin epitaxial layer,simultaneously grown in and over deep trenches and on the top surface onthe areas surrounding the deep trenches to achieve the super junctionstructure. The portion of the epitaxial layer in the trenches forms theepitaxial columns. The portion of the epitaxial layer grown over thedeep trenches and over the top surface on the areas surrounding the deeptrenches form the thin top epitaxial layer, in which trench MOSFET cellsare formed. Both portions of the epitaxial layer may be grown at thesame time as a single epitaxial layer. The trench gates of thetransistor cells are further shielded with doped shielding regionsimplanted through the trench gates into the drift region below the gatethus forming self-aligned dopant shielding regions to shield thesensitive gates during the voltage breakdown therefore the abovediscussed difficulties and limitations are resolved. The doped shieldingregions reduce the peak electric field at the trench gates; they alsoreduce the impact ionization rate and increase the breakdown voltage.The resulting structure has improved reliability and stability ofelectrical parameters. The doped shielding regions are formed under theaccumulation region below the trench gates, and do not need to contactthe trench gates. An additional dopant layer below the gate trenchhaving the same conductivity type as the accumulation region can ensurethat the doped shielding region does not contact the gate trench, whichmay allow more current to pass through when the device is switched on.

It is another aspect of this invention that the structure andconfiguration of the super-junction structure as that disclosed in thisinvention may be implemented with flexibly adjustable ranges of thetargeted breakdown voltages. The manufacturing processes are simplifiedand can be conveniently manufactured with standard processing usingstandard processing modules and equipment. The manufacturing processesare further simplified because the transistor portion of the structure,e.g., the trench gate double-diffused metal oxide semiconductor (DMOS),is self-aligned. Therefore, the above discussed technical difficultiesand limitations can be resolved.

Specifically, it is an aspect of the present invention to provide a newand improved device structure and manufacturing method to form theepitaxial layer in deep trenches with a thin top epitaxial layer portioncovering over the top surface of the device. A portion of this epitaxiallayer also serves as the body region of the MOSFET (p-type in the caseof n-channel MOSFET). Furthermore, the MOSFET cells are formed in thistop thin epitaxial layer as trench MOSFET. Trench gates are openedthrough the top thin epitaxial layer with optional trench sidewalls andtrench bottom dopant implant zones to eliminate the sensitivity of thechannel performance that may be affected by the depth of the trenchgates and the dopant concentration of the epitaxial layer. A pluralityof doped shielding regions are also implanted through the gate trenchesinto the drift region beneath the gates before the gate trenches arefilled with a gate polysilicon layer. The doped shielding regions are ofthe same conductivity type as the body region of the MOSFET and functionas gate shielding dopant regions which are self-aligned with the gatetrenches. The doped shielding regions may be floating islands, or theymay be tied (biased) to the epitaxial layer in the deep trenches andthus tied to the body region. Typically, floating islands are lessdesirable because they trap charges and make the device drift; thetrapped charges also slow electrical transitions since the trappedcharges take time to diffuse out. The performance of the transistorcells can be well controlled and adjusted by simplified and convenientprocessing steps. The super-junction configurations disclosed in thisinvention is further scalable for broad ranges of applications.

It is another aspect of the present invention to provide a new andimproved device structure and manufacturing method to form the powertransistor cells on a thin top layer formed as an epitaxial layercovering over the deep trenches and over the top surface surrounding andabove the deep trenches. Ion implantation (having an oppositeconductivity type as the epitaxial layer filling the deep trenches)through the deep trench sidewalls may adjust the dopant concentration ofthe drift regions surrounding the deep trenches to adjust and controlthe device performance parameters including the charge balance, Rdsonand breakdown voltage. The ion implantation thus provides charge controlto further adjust and fine tune the performance of the semiconductorpower devices for different types of applications.

It is another aspect of the present invention to provide new andimproved device structure and manufacturing method to form the powertransistor cells with shallow trench gates on thin top P-epitaxial layercovering over vertical trenches on the top surface surrounding areasabove the vertical trenches. Flexible device channel performance can beadjusted and implemented with a trench bottom dopant implant andsidewall dopant implants. The sidewall dopant implants and the trenchbottom implant are applied to compensate the P-epitaxial and to assureappropriate accumulation and the channel regions. Before the gatetrenches are filled with the polysilicon gate layer, an ion implant iscarried out through the bottom surface of the gate trenches. Thevertical implantation is applied to form the gate shielding dopantregions to shield the sensitive trench gates during the voltagebreakdown.

It is another aspect of the present invention to provide new andimproved device structure and manufacturing method to form the powertransistor cells with deeper trench gates in a thin top layer formed asan epitaxial layer covering over epitaxial columns and on the topsurface surrounding areas above the epitaxial columns. The trench gatespenetrate through the top thin epitaxial layer and extend into thesubstrate regions thus a trench bottom dopant implant for connecting tothe accumulation region may no longer be necessary. The trench gates arestill shielded by the gate-shielding dopant regions implanted throughthe bottom surfaces of the gate trenches to form aligned dopant regionsfor shielding the sensitive trench gates during a voltage breakdown. Atrench bottom dopant implant may still be used to ensure that thegate-shielding dopant regions do not contact the gate trenches.

Briefly in a preferred embodiment this invention discloses asemiconductor power device that includes a semiconductor substrateincludes a plurality of deep trenches. An epitaxial layer fills the deeptrenches; the epitaxial layer further includes a simultaneously growntop epitaxial layer covering areas above a top surface of the deeptrenches and over the semiconductor substrate. The epitaxial layer is ofan opposite conductivity type as the semiconductor substrate. Aplurality of trench MOSFET cells are formed in the top epitaxial layerwith the top epitaxial layer acting as the body region and thesemiconductor substrate acting as the drain region whereby asuper-junction effect is achieved through charge balance between theepitaxial layer in the deep trenches and regions in the semiconductorsubstrate laterally adjacent to the deep trenches. Each of the pluralityof trench MOSFET cells further includes a trench gate and agate-shielding dopant region disposed below and substantiallyself-aligned with each of the trench gates for each of the trench MOSFETcells for shielding the trench gate during a voltage breakdown. In anexemplary embodiment, each of the trench gates of the trench MOSFETcells is opened through the top epitaxial layer and filled with a gatedielectric material and a gate conductor material. In another exemplaryembodiment, each of the trench gates of the trench MOSFET cells isthrough the top epitaxial layer and penetrating into a top portion ofthe semiconductor substrate having a gate trench depth greater than orequal to a thickness of the top epitaxial layer and the trench gate isfilled with a gate dielectric material and a gate conductor material. Inanother exemplary embodiment, the trench gate further comprises gatesidewall dopant regions surrounding sidewalls of the trench gate and agate-bottom dopant region below the trench gate, wherein the gatesidewall dopant regions and gate-bottom dopant regions are of sameconductivity type as the semiconductor substrate. In another exemplaryembodiment, the semiconductor substrate further includes regionssurrounding the deep trenches having a lateral doping concentrationgradient with the concentration gradually decreasing from regionsimmediately next to sidewalls of the deep trenches. In another exemplaryembodiment, each of the MOSFET transistor cells further having gatesidewall dopant regions surrounding sidewalls of the trench gate and agate-bottom dopant region below the trench gate wherein the gatesidewall dopant regions and gate-bottom dopant regions are of a sameconductivity type as the semiconductor substrate. In another exemplaryembodiment, the drain contact dopant region surrounding a bottom portionof the deep trenches near a bottom surface of the semiconductorsubstrate for connecting to a drain electrode. In another exemplaryembodiment, the semiconductor power device further includes a bottommetal layer constituting a drain electrode contact the drain contactdopant region. In another exemplary embodiment, the trench gates of thetrench MOSFET cells and the deep trenches filled with the epitaxiallayer therein further configured as stripes with the gate-shieldingdopant regions disposed below stripes of the trench gates as floatingdopant regions. In another exemplary embodiment, the trench gates of thetrench MOSFET cells further configured as stripes with tabs extendingtoward the deep trenches filled with the epitaxial layer therein forelectrically connecting the gate-shielding dopant regions under theextended trench gates to a body region of the transistor cells throughthe epitaxial layer disposed in the epitaxial trenches. In anotherexemplary embodiment, the trench gates of the MOSFET transistor cellsfurther configured as stripes with offset tabs extending alternately onopposite sides of the trench gates toward the deep trenches filled withthe epitaxial layer for electrically connecting the gate-shieldingdopant regions under the extended trench gates to a body region of thetransistor cells through the epitaxial layer disposed in the deeptrenches.

This invention further discloses a method for forming a semiconductorpower device on a semiconductor substrate. The method includes a step ofa) providing a semiconductor substrate; b) opening a plurality of deeptrenches in the semiconductor substrate and growing an epitaxial layerfor filling the deep trenches and covers a top surface of thesemiconductor substrate with a top epitaxial layer, wherein the portionsof the epitaxial layer in the epitaxial deep trench and the topepitaxial layer are simultaneously grown as a single layer and whereinthe epitaxial layer is of an opposite conductivity type as thesemiconductor substrate; and c) forming a plurality of trench MOSFETcells in the top epitaxial layer by opening a plurality of trench gatesand implanting a plurality of gate-shielding dopant regions below thegate trenches for shielding trench gates of the transistor cells duringa voltage breakdown of the semiconductor power device and the topepitaxial layer acting as the body region and the semiconductorsubstrate acting as the drain region, whereby a super-junction effect isachieved through charge balance between the portions of the epitaxiallayer in the deep trenches and the portions of the semiconductorsubstrate lateral to the deep trenches. In an exemplary embodiment, themethod further includes a step of implanting through the sidewalls ofthe deep trenches with dopants of a first conductivity type to form alateral concentration gradient in regions of the semiconductor substratebetween the deep trenches and adjusting the device performance of thesemiconductor power device by adjusting the deep trench sidewallimplant. In another exemplary embodiment, the method further includes astep of controlling a thickness of the top epitaxial layer by adjustinga width of the deep trenches. In another exemplary embodiment, themethod further includes a step of implanting the sidewalls and bottom ofthe gate trenches a dopant of a same conductivity type as thesemiconductor substrate. In another exemplary embodiment, the step ofproviding a semiconductor substrate includes a step of providing asingle layer semiconductor substrate and wherein the step of opening aplurality of deep trenches comprises a step of opening a plurality ofdeep trenches in the single layer semiconductor substrate. In anotherexemplary embodiment, the step of providing a semiconductor substratecomprising a step of providing a bottom substrate and growing a topsubstrate layer on top of the bottom substrate, wherein the topsubstrate layer is of a same conductivity type as the bottom substrate.In another exemplary embodiment, the method further includes a step ofheavily doping the bottom of the deep trenches to form a drain contactregion before the step of growing the epitaxial layer; and backgrindingthe substrate to expose the drain contact region. In another exemplaryembodiment, the method further includes a step of performing a partialCMP to a top surface of the epitaxial layer to smooth the top surfacebefore the step of forming the plurality of trench MOSFET cells.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1B are cross sectional views for showing conventionalvertical power device configurations manufactured by conventionalmethods.

FIGS. 1C-1 to 1C-3 are cross sectional views for showing floatingislands formed in the bulk of the epitaxial layer not aligning with thegate or gate trenches.

FIG. 1D is a cross sectional view for showing dopant regions below thegate trenches connected to the trenches.

FIGS. 2 to 8 are cross sectional views of high voltage power deviceswith super junction structure as different embodiments of thisinvention.

FIGS. 9 to 12 are top views for illustrating different layoutconfigurations for arranging the trench-shielding dopant regions.

FIGS. 13A to 13N are a cross sectional views to illustrate processingsteps of this invention to manufacture high voltage power device similarto FIG. 3 with super junction structures and self alignedtrench-shielding dopant regions.

DETAILED DESCRIPTION OF THE METHOD

Referring to FIG. 2 for a cross sectional view of a MOSFET device 100that illustrates the new concepts including the new structural andmanufacturing features of this invention. The details of the MOSFETdevice 100 will be further described and explained in FIG. 3 below. TheMOSFET device 100 is supported on a substrate 105 that includes an N+doped bottom region 120 functioning as drain contact region dopedthrough deep trenches 130 (after back-grinding, as shown in FIG. 3below) that is filled with an epitaxial layer therein. The substrate 105may include a top portion 125 in which the deep trenches 130 are formed.For the example of an n-channel MOSFET, the substrate 105 is n-type, andthe epitaxial layer in the deep trenches 130 is p-type. The MOSFETtransistor cells are supported on the single thin epitaxial layerfilling in epitaxial column trenches 130 and covering over the topsurface surrounding the P-epitaxial columns that has the P-epitaxialfilling in the column trenches. The thin P-epitaxial layer portion overthe top also serves as the body regions surrounding the trench gate 145filled with gate polysilicon therein. The P-body regions 150 furtherencompass the source regions 155 surrounding the trench gates 145. Thetrench gates 145 are padded with a gate oxide layer 140, filled withpolysilicon and covered with an insulation layer 160 with contactopenings to allow a source contact metal to contact the source-bodyregions between the trench gates 145. The trench gates 145 are furthershielded with gate-shielding dopant regions 144 implanted through thegate trenches before the trenches are filled with the gate polysilicon.The gate-shielding dopant regions 144 are therefore self-aligned withthe trench gates 145. The gate-shielding region 144 have the sameconductivity type as the epitaxial layer filling in epitaxial columntrenches 130.

The device as shown in FIG. 2 provides a single thin epitaxial layer toform the trench gates with gate polysilicon filled in the trenchesopened therethrough. The new configuration achieves super-junctionperformance, i.e., performance below the “Johnson Limit”, with breakdownvoltage independent of the thickness of the epitaxial layer grown on thestarting substrate. It is the depth of the trench into the semiconductorsubstrate as well as the charge balance of the epitaxial column trencheswith the substrate regions thereinbetween, which set the breakdownvoltage. The thickness of the epitaxial silicon growth is only afunction of the width of the deep trenches which are etched in thesilicon substrate. Conventional devices do not have this flexibility andmust grow epitaxial layers as drift regions which have a thicknessproportional to the desired breakdown voltage.

The structure shown is flexibly scalable and the device can bemanufactured by applying simple manufacturing technologies. For example,a device capable of achieving a low specific resistance below theJohnson Limit, over a wide range of breakdown voltage (200V to 900V forexample) can be realized by using a single epitaxial silicon layergrowth of a few microns, and a single trench etch with a depthproportional to the desired breakdown voltage (about 10-15 micronfor >200V, about 40-50 micron for >600V and about 70-90 micronfor >900V). Furthermore, the transistor portion of the device supportedon the top portion of the epitaxial layer 130 is structured astrench-gate DMOS device wherein the device configuration is self-alignedand can be conveniently and easily manufactured. The sensitive trenchgate 145 portion of the device is formed away from seams which may formabove the deep trenches 130, which improves the reliability of thedevice. This feature can also avoid the use of an additional CMP step.

Referring to FIG. 3 for a cross sectional view of a MOSFET device 100with the new concepts and basic structures illustrated in FIG. 2 andmanufactured according the processes described in FIGS. 13A to 13Nbelow. The MOSFET device 100 is supported on an N type substrateincludes an N+ doped bottom region 120 functioning as drain contactregion immediately above and in contact with a bottom drain electrode110. The drain contact regions 120 are doped through deep trenchescontaining epitaxial layer 130. Each of these deep trenches is filledwith a P-epitaxial layer filling the trenches and covering the topsurfaces surrounding and above the trenches. The MOSFET transistor cellsare supported on the single thin P-epitaxial layer that fills inepitaxial column trenches 130 and covering over the top surfacesurrounding the P-epitaxial columns. The thin P-epitaxial layer over thetop is formed with P-body regions 150 surrounding the trench gates 145with gate polysilicon filling in the trenches opened through the topepitaxial layer 130. The P-body regions further encompass the sourceregions 155 surrounding the trench gates 145. The trench gates 145 arepadded with a gate oxide layer 140 and covered with an insulation layer160 with contact openings to allow a source contact metal 170 over ametal barrier layer 165 to contact the source-body regions between thetrench gates 145. The trench gates 145 are further shielded with p-typegate-shielding dopant regions 144 implanted through the gate trenchesinto the N-substrate regions 125 before the gate trenches are filledwith the gate polysilicon. The gate-shielding dopant regions 144 protectthe sensitive gates 145 during a voltage breakdown of the MOSFET device.The N substrate regions 125 surrounding the P-epitaxial columns 130 maybe implanted with N-dopant through the deep trench 130 sidewalls forachieving to create a lateral doping concentration gradient forachieving an N-column charge control.

The Super-Junction effect, or charge balance, is achieved by ensuringthat the electrical charges of the P-epitaxial layer filled in thetrenches is laterally balanced, i.e., balanced along the distance thatis perpendicular to the drain current flow in the n-type drift region125 of the vertical MOSFET structure, so that they deplete when theMOSFET is in the off state. In other words, the electrical charges ofthe P-epitaxial layer filled in the trenches are substantially equal tothe electrical charges of the N-drift region adjacent to the N-substratewithin the manufacturing tolerances. The electrical charges in theN-drift region can be controlled and adjusted by controlling either thedoping of the N-substrate, or the doping of the N-substrate plus anyadditional N-dopant ions which may be implanted in the sidewalls of thedeep trenches. For ideal operation, the P=N=1E12 atoms per squarecentimeter is the target charge. The more flexible the control of theelectrical charges in the manufacturing processing by controlling theimplant dosages, implant annealing, substrate doping concentration,epitaxial dopant concentration, trench depth, width and shape, and theparameters of other processing steps, the more the structure of thedevice can be optimized and fine tuned to achieve a lower specificresistance for a given breakdown voltage.

The MOSFET transistor cells further include N type dopant implantregions 135-S along the gate sidewalls and N type dopant implant regions135-B below the gate trench bottom. The sidewall and bottom dopantimplant regions surrounding the gates 145 may be applied to eliminatethe sensitivity of the channel of the MOSFET device relative to thetrench depth and the P-epitaxial dopant concentrations. This embodimentwith the new structure is based on the consideration of forming ahigh-performance MOSFET structure inside the P-epitaxial layer. Theepitaxial layer is grown with minimal or no etch-back of the P-epitaxiallayer at all. For a MOSFET to function, it must have a source of thesame conductivity as the drain, and a body which has an oppositeconductivity, as well as an accumulation region which connects thechannel to the drain. When a trench-gate vertical MOSFET structure isrealized, the source is at the top with the channel formed in a bodyregion below the source and along the sidewalls of the gate trenches. Anaccumulation region must be formed between the body region and thedrain. For a high-voltage structure with the new configuration asdisclosed in this invention, it would be difficult to form ahigh-performance vertical trench-gate MOSFET when the P-epitaxial grownon the top horizontal surfaces of the N-substrate is very thick. With athick P-epitaxial layer, the gate trenches would have to be deep inorder to reach through to the N-drift drain region. A deep trench,combined with a thick P-body region, will result in a low-performancevertical DMOS structure because of the resulting long channel and highchannel resistance. Therefore, in the embodiments of this new inventionwhich deal with P-epitaxial layers which may have a greater thicknessbetween one to three urn relative to the typical depth of gate trenchesgenerally in a range between 0.8 to 1.5 um, additional dopant implantsin the gate trench sidewalls and bottom are carried out. The additionaldopant implants are to compensate the P-epitaxial region in theaccumulation and drain regions in proximity to the gate trenches, inorder to realize high performance vertical trench-DMOS devices, withshortened channel lengths. Therefore, the addition of the tilted andnon-tilted implants in the gate trench prior to the fabrication of theMOSFET device is the gate trenches enables high-performance trench-gateMOSFET devices independent of the P-epitaxial layer thickness and dopantconcentrations in these regions. An n-type dopant implant 135-B at thebottom of the gate trench may also serve to ensure that the gateshielding regions 144 do not contact the gate trenches 145.

It should be noted that the embodiment of FIG. 3 shows a gate trenchwhich reaches through the P-epitaxial layer, and therefore, theadditional N-type implants 135-S, 135-B, may be selected to optimize theperformance of the MOSFET without the need to compensate completely theP-doped region, i.e., the P-epitaxial layer, on the gate trenchsidewalls. Implant species can be preferably phosphorus as well asArsenic or Antimony. Energy can be in the 50 KeV to 200 KeV range. Tiltangle should be zero for the bottom implant, and +/−5 to 15 degrees forthe sidewall implants. Dose can be in the 1E11 to 1E13 range. Anadditional p-type body implant may be performed to form body region 150and ensure that a channel region remains along the trench gate 145sidewalls.

FIG. 4 is a cross sectional view to show an alternate embodiment of aMOSFET device similar to that shown in FIG. 3 except that the sidewallsof N-substrate regions 125′ are not implanted with N dopant to achievethe charge control function through the manufacturing processes. Thisembodiment does not require the additional N-dopant regions forincorporation in the sidewalls of the deep trenches, because it isassumed that the doping concentration of the starting N− substrate isadequate to ensure charge balance with the grown P type epitaxial layerin the deep trenches. The doping concentration of the starting N−substrate is adequate when the actual value of the doping concentrationcan achieve the necessary charge balance, i.e., achieving the goal ofhaving approximately absolute value of the N charge=P charge=1E12atoms/cm^2. A dopant implant to carry out the charge control is notnecessary when the substrate concentration can achieve the chargebalance goals within the desired tolerance limits. (for example, whenthe occurrences of the N-substrate having adequate dopant concentrationis better than +/−10% repeatability in the manufacturing processes).

FIG. 5 is a cross sectional view to show an alternate embodiment of aMOSFET device similar to that shown in FIG. 3 except that the MOSFETdevice does not include a sidewall and trench bottom dopant implantregions 135-B and 135-S shown in FIG. 3. When the trench gates 145 havea greater depth and extend below the epitaxial layer 130 into thesubstrate region 125, the requirement for applying the trench sidewalland trench bottom dopant implant regions to eliminate the channelsensitivity to the depth of the trench gates are no longer necessary.

FIG. 6 is a cross sectional view to show an alternate embodiment of aMOSFET device similar to that shown in FIG. 3 except that the MOSFETdevice has shallower trench gates with a depth shallower than theepitaxial layer. The MOSFET device includes a gate trench sidewall andgate trench bottom dopant implant regions 135-S and 135-B, respectively,to compensate the P-epitaxial layer 130 and to ensure that there areappropriate accumulation and channel regions. This embodiment is basedon a configuration that the MOSFET device has thick P-epitaxial layers,or shallow gate trenches, or a combination of both. The gate trenches donot reach to the N drain region. In order to ensure proper and efficienttransistor operations, the lower portion of the gate trenches must bedoped as an N dopant region 135-B to ensure that there will be anaccumulation region to connect the drain to the active channel formed inthe body region along the sidewalls of the gate trenches.

Conventional wafers have a heavily doped substrate, with lightly dopedlayer on top. However, the devices in FIGS. 2-6 were made from a plainwafer that did not initially have an epi-layer. This can save aconsiderable amount of the wafer costs, but requires extra steps ofdoping the bottom through the deep trench and back grinding the wafer.Alternatively, the device in FIGS. 7-8 uses a conventional wafer with aheavily doped N+ bottom substrate 121, and a less heavily doped N-typetop substrate layer 126 grown over the N+ bottom substrate 121. In aconventional wafer, this N-type top substrate layer 126 is known as anepitaxial layer, however to avoid confusion, it is referred to in thisapplication as a top substrate layer. FIG. 7 is a cross sectional viewto show an alternate embodiment of a MOSFET device similar to that shownin FIG. 3 except that the deep trenches 130 filled with P-epitaxiallayer is now located in the top substrate layer 126 and extended totouch the highly doped bottom substrate region 121. A separate draincontact region 120 of FIG. 3 formed by a separate dopant implant processis no longer required. Instead, a highly doped N+ bottom substrateregion 121 with an N-type top substrate layer 126 grown on top of it isused as the drain contact for this embodiment. The top substrate regionmay be thin compared to those of conventional wafers, thus saving cost.A backside grinding might not be required for this embodiment. A metaldrain electrode 110 may be formed beneath the highly doped bottomsubstrate region 121.

The drain contact dopant implant process at the bottom of the deeptrenches is eliminated, so the manufacturing processes are thereforesignificantly simplified.

FIG. 8 is a cross sectional view to show an alternate embodiment of aMOSFET device similar to that shown in FIG. 7 except that the deeptrenches 130 filled with P-epitaxial layer is now at a depth that isshallower than the N+ bottom substrate 121.

FIG. 9 is a top view for showing a stripe layout of the semiconductorpower device of this invention. The epitaxial deep trenches grown withthe epitaxial layer 130 are formed with stripe configuration The outlineof the epitaxial deep trenches 130 are shown in dashed lines. Thetransistor cells including the trench gates 145 padded with the gateoxide layer 140 surrounded by the source regions 155 encompassed in thebody regions 150 are also formed with a linear stripe layout. Theself-aligned gate-shielding dopant regions (not specifically shown here)are also formed as floating stripe below the trench gates 145.

FIG. 10 shows an alternate embodiment with a different transistor celllayout. Instead of manufacturing the gate-shielding dopant regions 144below the trench gate 145 as floating regions, the gateshielding-p-dopant regions 144 may be connected to the P-dopantepitaxial columns 130 below the body regions 150 by extending the trenchgate 145 as cross shape trench gates to the P-columns 130 regions insome parts of the transistor cells as shown in FIG. 10. FIG. 11 shows asimilar embodiment with the exception that the extended trench gates 145have offset tabs 145-TB to reduce the drain-source on resistance Rdson,and to make improve the manufacturability of the device. (cross-shapedgate trenches may have voiding issues when filled). FIG. 12 shows thesame layout as FIG. 11, but illustrates how the gate-shielding dopantregion tabs 144-TB self aligned under the gate trenches 145, and arediffused out to contact the p-doped columns 150. The gate-shieldingdopant regions 144 are implanted under the main gate stripe 145 andunder the gate tabs 145-TB perpendicular to the main gate stripe. Thep-shield dopant regions 144 electrically contact the p-body 150 regionsthrough the connection regions intersecting between the gate-shieldingdopant region tabs 144-TB and the p-epitaxial columns 130. The offsetlayout has reduced impact to the channel width. The offset tabs may alsoachieve better distributed current flows by allowing current flow on theother side of the trench gate tabs 145-TB.

Referring to FIGS. 13A to 13N for a serial of side cross sectional viewsto illustrate the fabrication steps of a charge balanced semiconductorpower device as that shown in FIG. 3. FIG. 13A shows a starting siliconsubstrate includes an N substrate 205 having a resistivity approximately10 ohm/cm. The N substrate 205 initially has no epitaxial layer. A hardmask oxide layer 212 is deposited or thermally grown with a thickness of0.1 to 1.5 micrometer. Then a trench mask (not shown) with a criticaldimension (CD) in a 1 to 5 microns range, is applied to carry out anoxide etch to open a plurality of trench etching windows followed byremoval of the photoresist. A silicon etch is carried out to open deeptrenches 214 with a depth of 40 to 50 microns for devices operated at avoltage of about 650 volts. Depending on the type of etcher and etchchemistries, photoresist only mask may also be used to pattern and openthe trench as well instead of using the hard mask oxide layer 212 asshown. The trench opening may be in the 1 micron to 5 microns rangepreferably 3 microns for most applications (the trench opening beingdefined by the trench mask mentioned earlier). Then a wafer cleanprocess is performed. In FIG. 13B, a conformal oxide layer 215 is formedby either an oxide deposition or thermal growth process. Then anoptional RIE anisotropic etch is carried out to clear the oxide from thebottom of the trench bottom surface if the oxide layer is thicker on thebottom surface. The thickness of the oxide layer 215 is between 0.015 to0.1 micron when the process does not include the optional RIE step, andthe layer thickness of the oxide layer 215 is between 0.1 to 0.4micrometers when the processes include the optional RIE step. A draincontact implant is performed by implanting N+ ions along a zero tiltangle relative to the sidewalls of the trenches, i.e., a verticalimplant, having an implant dosage greater than 1E15 to form the draincontact regions 220 immediately below the deep trenches 214. The draincontact region 220 may be implanted with N-type ions such as phosphorusor arsenic ions. The oxide layer 215 along the sidewalls protects thesidewalls from the high dosage of the drain contact implant.

In FIG. 13C, a trench sidewall implant is carried out with N-type ionssuch as phosphorous ions to set the doping concentration in the Nregions. An implant with tilted angle and rotated operations areperformed with a dosage of 5E11 to 2E13 and a tilt angle of five tofifteen degrees are carried out to form the N-regions 225 between thetrenches depending on the trench depth. In FIG. 13D, a high temperatureanneal operation at 1050 to 1200 degrees Celsius for 30 to 60 minuteswith low oxygen (O2) and/or N2 is applied to diffuse the N+ draincontact region 220 and also to laterally diffuse the sidewall implantN-regions 225. The N-regions 225 now form a lateral N-type concentrationgradient, with the concentration being greatest near the deep trenchside walls. The sidewall implants may be used to adjust the N-typeconcentration of the regions of the substrate 205 that are along alateral direction relative to the deep trenches in order to achieve acharge balance (for super-junction effect) with the P-epitaxial layer230 (about to be grown). Alternatively to the sidewall implants, thesubstrate 205 can be initially formed with the required N-typeconcentration to achieve the super-junction effect. In FIG. 13E, theoxide layers 212 and 215 are etched off and a P-epitaxial layer 230 witha P dopant concentration of 1E15 to 1E16 or higher, is grown (dependingon the desired breakdown voltage). The thickness of the P-epitaxiallayer 230 is sufficient to fill the trenches 214. For a trench 214 witha width of about 3 microns, the thickness of the epitaxial layer 230 isapproximately 1.5 to 2.0 microns over the top of N-region 225. In FIG.13F, an oxide layer is deposited with a thickness of about 0.5 to 1.5microns as hard-mask layer 228 followed by applying a gate trench mask(not shown) to etch the hard-mask oxide layer 228 then removing thephotoresist. The width of the gate trench may be of the order of 0.4 to1.5 micron typically. A silicon etch is carried out to etch the trenchgate openings 232 through the P-epitaxial layer 230 with a trench depthof about 1 to 2.5 microns that may penetrate through the P-epitaxiallayer 230 into the N-dopant regions 225 between the epitaxial columns230 deposited into the trenches 212. The process is then followed withwafer cleaning and optionally a round hole is etched to smooth the gatetrench profile followed by another wafer cleaning process.

In FIG. 13G, the oxide hard mask 228 is removed followed by depositing athin screen layer 234 covering the sidewalls and the bottom surface ofthe gate trenches 232. A deep P-type implant with boron (B11) ions withan energy between 200 to 600 KeV and a dose between 1E12 to 1E13 iscarried out with zero tilt angle to form the gate-shielding P-dopantregions 244 below the gate trenches 232 in the N-doped columns 225. InFIG. 13H, an optional n-type gate trench sidewall-implant with a tiltangle between +/−5 to 7 degrees implanting angle is carried outoptionally to compensate the P-epitaxial layer 230 followed by a n-typegate trench bottom implant with zero tilt angle to compensate theP-epitaxial layer 230 if the gate trench 232 is too shallow, or toensure that the gate-shielding P-dopant regions 244 do not contact thegate trenches 232. The implants into the gate trench sidewalls and thebottom surface to form the sidewall and bottom dopant regions 235-S and235-B respectively eliminate the sensitivity of the channel of theMOSFET device relative to the depth of the trench gates and the dopingconcentration/thickness of the P-epitaxial layer 230. In FIG. 13I, thescreen oxide layer 234 is removed and a gate oxide layer 240 is grownhaving a thickness of 0.01 to 0.1 micron depending on the device voltagerating. A gate polysilicon layer 245 is deposited into the gate trenches232. The gate polysilicon layer 245 is preferably performed with in-situN+ doping; the polysilicon layer 245 is doped by ion implantation ordiffusion, if the polysilicon layer 245 is not in-situ doped. The gatepolysilicon layer 245 is etched back from the top surface surroundingthe trenched gates 245.

In FIG. 13J, an optional body mask (not shown) is applied to carry out abody implant with boron for a NMOS device having a dosage ranging from3E12 to 1E14 followed by a body drive process at a temperature of 1000to 1150 degree Celsius to form the P-body regions 250 in the epitaxiallayer 230 surrounding the trench gates 245. The body implant allows forgood contact to the body regions and also ensures a MOSFET channelregion is preserved above the gate sidewall implants 235-S. In FIG. 13K,a source dopant implant is carried out. A source implant mask (notshown) is optionally applied to protect the locations to form the P-bodycontact. The source implant is carried out with source dopant ions suchas arsenic ions with a dosage about 4E15 at an energy about 70 KeV at azero degree tilt followed by a source anneal operation at a temperatureof approximately 800 to 950 degrees Celsius to diffuse the sourceregions 255. In FIG. 13L, a dielectric layer 260 that by a LowTemperature Oxide deposition (LTO) and borophosphosilicate glass (BPSG)layer 260 is formed on the top surface followed by a BPSG flow process.Then a contact mask (not shown) is applied to carry out an oxide etch toetch the contact openings through the BPSG layer 260. A P+ body contactimplant is carried out as an optional step followed by a reflow afterthe body contact implant. In FIG. 13M, a barrier metal deposition iscarried out to cover the top surface with a barrier metal layer 265followed by a thick metal deposition to form the source metal layer 270.A metal mask (not shown) is applied to etch and pattern the source metal260 and gate metal (not shown). The processes are completed with thedeposition of dielectric layers to passivate the device surface, and thepatterning of the passivation layer to form the bond pad openings (notshown). A final alloy can then be performed. For the sake of brevity,these standard manufacturing processes are not specifically describedhere. In FIG. 13N, a backside grinding operation is carried out toremove the low doped portion of the substrate 205 from the bottomsurface of the substrate then a back metal layer 210 is formed tocontact the drain region 220 where the dopant concentration is higher.The back metal layer 210 may be formed by a deposition of TiNiAg layerdirectly on the backside of the wafers (below the drain contact region220). The back grinding operation has a thickness control with a fewmicrons and even down to one micron thus enables a reliable backsidecontact to form the drain electrode layer 210 to contact the N+ draincontact regions 220.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. By way of example, although theabove embodiments show a n-channel device, this invention is easilyapplicable to p-channel devices by switching the conductivity types ofthe semiconductor regions. Accordingly, it is intended that the appendedclaims be interpreted as covering all alterations and modifications asfall within the true spirit and scope of the invention.

I claim:
 1. A semiconductor power device comprising: a semiconductorsubstrate of a first conductivity type comprises a bottom substratelayer having a higher dopant concentration than a top substrate layerwherein a plurality of deep trenches are opened in the top substratelayer; an epitaxial layer filling said deep trenches, the epitaxiallayer further including a top epitaxial layer disposed above the deeptrenches and covering a top surface of the top substrate layer, whereinthe epitaxial layer having a second conductivity type opposite the firstconductivity type; a plurality of trench MOSFET cells disposed in saidtop epitaxial layer with the top epitaxial layer constituting a bodyregion and the semiconductor substrate constituting a drain regionwherein the epitaxial layer in the deep trenches is charge-balanced withthe top substrate layer surrounding the deep trenches; and each of saidplurality of trench MOSFET cells further including a trench gate filledwith a gate polysilicon therein and padded with a gate insulation layer.2. The semiconductor power device of claim 1 further comprising:gate-shielding dopant regions of the second conductivity type disposedat a distance below a bottom surface of the trench gates.
 3. Thesemiconductor power device of claim 1 further comprising: dopant regionsof the first conductivity type surrounding and immediate below a bottomsurface of each of the trenched gates.
 4. A semiconductor power devicecomprising: a semiconductor substrate of a first conductivity typecomprises a bottom substrate layer having a higher dopant concentrationthan a top substrate layer wherein a plurality of deep trenches areopened in the top substrate layer; an epitaxial layer filling said deeptrenches, the epitaxial layer further including a top epitaxial layerdisposed above the deep trenches and covering a top surface of the topsubstrate layer, wherein the epitaxial layer having a secondconductivity type opposite the first conductivity type; a plurality oftrench MOSFET cells disposed in the top epitaxial layer functioning asbody regions surrounding a trench gate filled with a gate polysiliconand padded with a gate insulation layer having a trench gate depthshallower than the top epitaxial layer wherein the body region furtherencompassed dopant regions of the first conductivity type near a topsurface of the semiconductor substrate functioning as a source regionfor each of the MOSFET cells and wherein the epitaxial layer of thesecond conductivity type filled in the deep trenches is charge-balancedwith the semiconductor substrate of the first conductivity typesurrounding the deep trenches; and a gate-shielding dopant regioncomprising a dopant region of the second conductivity type disposedbelow and substantially aligned with each of the trench gates for eachof the trench MOSFET cells.
 5. The semiconductor power device of claim 4wherein: the gate-shielding dopant regions are disposed at a distancebelow a bottom surface of the trench gates.
 6. The semiconductor powerdevice of claim 4 further comprising: dopant regions of the firstconductivity type surrounding and immediate below a bottom surface ofeach of the trenched gates.
 7. The semiconductor power device of claim 1wherein: the semiconductor substrate is an N-type semiconductorsubstrate and the epitaxial layer filling the deep trenches and havingthe top epitaxial layer is a P-type epitaxial layer.
 8. Thesemiconductor power device of claim 1 wherein: the semiconductorsubstrate further comprising a drain electrode contact layer having aheavier dopant concentration than the semiconductor substrate forcontacting a drain electrode disposed on a bottom surface of thesemiconductor substrate.
 9. The semiconductor power device of claim 1wherein: the semiconductor substrate is an N-type semiconductorsubstrate and further comprising a N+ dopant layer as a drain electrodecontact layer for contacting a drain electrode disposed on a bottomsurface of the semiconductor substrate.
 10. The semiconductor powerdevice of claim 1 further comprising: a top insulation layer coveringover the trench gates of the MOSFET cells and exposing the source andbody regions for contacting a source metal covering over the topinsulation layer.
 11. The semiconductor power device of claim 1 furthercomprising: a top insulation layer covering over the trench gates of theMOSFET cells and exposing the source and body regions for contacting ametal barrier layer covering over the top insulation layer; and a sourcecontact metal layer covering over and contacts the metal barrier layerto function as a top s urce electrode.
 12. The semiconductor powerdevice of claim 4 wherein: the semiconductor substrate is an N-typesemiconductor substrate and the epitaxial layer filling the deeptrenches and having the top epitaxial layer is a P-type epitaxial layer.13. The semiconductor power device of claim 4 wherein: the semiconductorsubstrate further comprising a drain electrode contact layer having aheavier dopant concentration than the semiconductor substrate forcontacting a drain electrode disposed on a bottom surface of thesemiconductor substrate.
 14. The semiconductor power device of claim 4wherein: the semiconductor substrate is an N-type semiconductorsubstrate and further comprising a N+ dopant layer as a drain electrodecontact layer for contacting a drain electrode disposed on a bottomsurface of the semiconductor substrate.
 15. The semiconductor powerdevice of claim 4 further comprising: a top insulation layer coveringover the trench gates of the MOSFET cells and exposing the source andbody regions for contacting a source metal covering over the topinsulation layer.
 16. The semiconductor power device of claim 4 furthercomprising: a top insulation layer covering over the trench gates of theMOSFET cells and exposing the source and body regions for contacting ametal barrier layer covering over the top insulation layer; and a sourcecontact metal layer covering over and contacts the metal barrier layerto function as a top source electrode.